Wireless user terminal and system having signal clipping circuit for switched capacitor sigma delta analog to digital converters

ABSTRACT

A wireless user terminal ( 42 ) and system ( 40 ) implementing a mixed signal CODEC ( 100 ) including an improved sigma-delta ADC ( 18 ) which limits input signals into a switched capacitor configuration and avoids adding circuit overhead in the signal path is disclosed herein. This sigma-delta analog-to-digital converter ( 18 ), having an input signal and an output signal, includes a switch (sw 1 ), a clipping circuit ( 20 ), and a known sigma-delta ADC ( 34 ). It solves the clipping signal problem by limiting the signal right at the input of the sigma-delta ADC ( 34 ). The clipping circuit ( 20 ) couples to the switch (sw 1 ) and the sigma-delta ADC ( 34 ) for switching the voltage applied to the sigma-delta ADC between the input signal (v in ) and at least one threshold voltage (V n  and V p ).

FILED OF THE INVENTION

This invention relates generally to the field of electronic systems and,in particular, to a wireless user terminal and system having signalclipping circuits for switched capacitor sigma delta analog-to-digitalconverters included within audio codec systems.

BACKGROUND OF THE INVENTION

The codifier/decodifier (CODEC) is the algorithm that handles the codingand decoding of audio signals within an electronic system. Specifically,an audio CODEC is a custom mixed-signal core providing analog-to-digital(A/D) and digital-to-analog (D/A) conversion. A simple serial interfaceis used to exchange digital data (D/A input and A/D output) between theapplication specific integrated circuit (ASIC) and CODEC core. Prior artCODEC features delta-sigma A/D and D/A oversampled converters and lowpower dissipation.

A typical uplink channel for a mobile phone voiceband or audio CODECincludes a microphone, amplifier, sigma-delta analog-to-digitalconverter (ADC) and a digital filter coupled together on one chip. Thisfirst chip couples to a digital signal processor for processing thedigital signal received. Another chip includes a radio frequency (RF)modulator which is coupled to a last component that includes a RF poweramplifier. The signal is transmitted over an antenna to a downlinkchannel for the mobile phone voiceband CODEC.

Initially, the audio CODEC receives an analog voice signal through themicrophone and converts it to a digital signal. The digital signal isforwarded to a digital signal processor for processing. This signal istransmitted to a receiver. In the receiver, the digital signal isprocessed through the digital signal processor and forwarded to a D/Aconverter. The analog signal is fed to a speaker.

In most prior art CODECs, the sigma-delta ADC is scaled for a maximumoutput corresponding to the +3 dbm0 code of the pulse code modulation(PCM) data. The analog signal corresponding to this digital upscalevalue is far less than the maximum allowable dynamic range, whichusually is limited by the supply range. This fact could potentiallyoverload the A/D and consequently the digital filter. An FCC test,mandatory in the U.S., falls under this category. Once the digitalfilter overloads, internal clipping mechanisms prevent wrap around ofthe digital signal, thus creating a digital representation of atrapezoidal signal that contains harmonics with sufficient power toincrease the FM modulation depth.

First and second order sigma delta analog modulators are inherentlystable under large input level variations. Higher order modulators,however, can become unstable during the overload condition. Clipping theinput signal to a pre-determined safe operation level, prevents themodulator from going unstable, without having the need to recoverstability after the overloading condition is removed. In other cases,even inherently stable sigma-delta structures have to be protected by aclipping mechanism to prevent post digital filtering from generation ofa rail-to-rail digital representation of a quasi-square wave which canover-modulate the RF channel in a typical transmit CODEC channel forwireless applications.

Several implementations have been proposed to solve this problem. Mostof them deal with clipping the signal in a previous analog amplifierstage. One solution is provided in U.S. patent application Ser. No.09/351,610, which discloses a multiplexer amplifier having an analogoutput signal, a sigma-delta ADC having an input coupled to the analogoutput signal and a clipping circuit coupled to the input of the ADC forclipping the analog output signal. While this analog solution avoidssaturation and provides an effective clipping mechanism to prevent wraparound of the digital signal, it is prone to overshoot and settlingissues.

In present systems, however, the signal is fed to the A/D directly fromexternal sources, such as a microphone or an RF mixer. Accordingly, manyaudio CODECs no longer include the microphone and amplifier. Thus, thereis a need for a wireless user terminal and system that incorporate asigma-delta analog-to-digital converter (ADC) that is free of overshootand settling issues.

SUMMARY OF THE INVENTION

A wireless user terminal and system that implement a mixed signal CODECincluding an improved sigma-delta ADC limits input signals into aswitched capacitor configuration and avoids adding circuit overhead inthe signal path. Additionally, the improved sigma-delta ADCsubstantially reduces overshoot and settling problems common in userterminals. This improved sigma-delta ADC, having an input signal and anoutput signal, includes a switch, a clipping circuit, and a sigma-deltaADC. It solves the clipping signal problem in wireless user terminals bylimiting the signal right at the input of the sigma-delta ADC. Theclipping circuit couples to the switch and the sigma-delta ADC forswitching the voltage applied to the sigma-delta ADC between the inputsignal and at least one threshold voltage. When the input signal goesabove a prescribed upper threshold, the fixed threshold voltage isapplied to the sigma-delta ADC, which converts fixed threshold voltageinto a digital signal. Moreover, when the input signal goes below thatprescribed threshold, the incoming signal is applied to the sigma-deltaADC, which converts the incoming signal. In the alternative, when theinput signal goes below a prescribed lower threshold, the fixedthreshold voltage is applied to the sigma-delta ADC, which convertsfixed threshold voltage. Furthermore, when the input signal goes abovethat prescribed threshold, the incoming signal is applied to thesigma-delta ADC, which converts the incoming signal. Given thissolution, minimum power and area overhead exist.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings in which likereference numbers indicate like features and wherein:

FIG. 1 is a schematic of a known simplified input stage of a sigma deltamodulator;

FIG. 2 is a schematic of a signal clipping circuit in accordance withthe present invention;

FIG. 3 a is a diagram of the input voltage applied with respect to time;and

FIG. 3 b is a diagram of the clipped input voltage in accordance withthe present invention.

FIG. 4 illustrates a communications system that implements the signalclipping circuit of one embodiment of the present invention;

FIG. 5 illustrates a block diagram of a wireless user terminalimplemented in an embodiment of the present invention;

FIG. 6 illustrates a wireless user terminal block diagram thatimplements the signal clipping circuit according to an embodiment of thepresent invention;

FIG. 7 illustrates a wireless user terminal receiver block diagram thatimplements the signal clipping circuit according to an embodiment of thepresent invention;

FIG. 8 illustrates the transmitted spectra for TDMA (GSM) and CDMA(IS-95) systems; and

FIG. 9 illustrates a spectral definition of 2G and 3G cellularregulations.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A circuit is presented here, that clips the incoming signal topredetermined levels without disturbing the signal path and addinglittle overhead to the power and area requirements. In FIG. 1, analogclipping circuits 40 and 42 are coupled to the differential inputs ofthe sigma-delta A/D 26, to avoid overdriving the sigma-delta A/D 26. Theanalog clipping circuits 40 and 42 add minimum overhead in area andpower. For the preferred embodiment, the maximum allowable dynamic rangeat the input of the sigma delta A/D 26 is a minimum of 0.625 volts and amaximum 2.375 volts. The fully differential signal is 3.5 volts (+1.75volts to −1.75 volts). Each single ended signal is clipped at a low of0.625 volts (V_(RL)) and a high of 2.375 volts (V_(RH)). This clippingproblem solution adds a pre-amp to the signal path. The amplifier then,has to perform better than the noise specification of the channel whichimplies high current consumption and silicon area utilization. Thissolution adds a constraint to the external driving source since now theinput to the chip is not capacitively coupled anymore but rather has lowresistance.

A voiceband CODEC 18 having an improved sigma-delta A/D converter inaccordance with the present invention is shown in FIG. 2. The incomingsignal v_(in) is connected to the comparators C_(p) 24 and C_(n) 22(which can be designed for very low current since speed and offset arenot a primary concern), as well. Threshold voltages, V_(p) and V_(n)(which can be generated from the bandgap or derived from the supplyvoltage through a resistor/diode division), are coupled to comparators,C_(p) 24 and C_(n) 22, respectively. The incoming signal is sensed bycomparators C_(p) and C_(n), comparing the incoming signal with voltagesV_(p) and V_(n). Comparators C_(p) and C_(n) are connected to switches,sw_(p) and sw_(n), respectively for switching in voltage levels, V_(p)and V_(n), respectively. All three switches, sw₁, sw_(p) and sw_(n),couple to a sigma-delta ADC 34. Switch sw₁ couples to receive theincoming signal V_(in).

In operation, when incoming signal v_(in) rises above the thresholdvoltage V_(p), switch sw₁ opens and comparator C_(p) turns on, closingswitch sw_(p). Accordingly, the fixed voltage V_(p) is supplied to thesigma delta ADC 34. When the value of the signal goes below thethreshold voltage, comparator C_(p) shuts off, opening switch sw_(p).Simultaneously, switch sw₁ closes and incoming signal v_(in) is supplieddirectly to sigma-DAC 34.

When incoming signal v_(in) goes below threshold voltage V_(n), switchsw₁ opens and comparator C_(n) turns on, closing switch sw_(n).Accordingly, the fixed voltage V_(n) is supplied to the sigma delta ADC34. When the value of signal v_(in) rises above the threshold voltageV_(n), the comparator C_(n) shuts off, opening switch sw_(n).Simultaneously, switch sw₁ closes and incoming signal v_(in) is supplieddirectly to the sigma delta ADC.

FIG. 3 a displays the input signal v_(in), while FIG. 3 b shows theclipped input signal v_(clip) seen by the sigma-delta ADC 34. As shownin FIG. 3 b, when incoming signal v_(in) rises above the thresholdvoltage V_(p), switch sw₁ opens and comparator C_(p) turns on, closingswitch sw_(p). As a result, the voltage V_(clip) is equal to thethreshold voltage V_(p), When the value of the signal v_(in) goes belowthe threshold voltage V_(p), the comparator C_(p) shuts off, openingswitch sw_(p). Switch sw₁ closes and, as a result, voltage v_(clip)equals the incoming signal v_(in). When the incoming signal v_(in) goesbelow threshold voltage V_(n), switch sw₁ opens and the comparator C_(n)turns on, closing switch sw_(n). Accordingly, voltage v_(clip) equalsthe fixed voltage V_(n).

The signal clipping circuit for switched capacitor sigma deltaanalog-to-digital converter (ADC) of the present invention can be usedin a variety of telecommunication and other applications. Conveniently,the signal clipping circuit for improved sigma delta analog-to-digitalconverters can be implemented in wireless user terminals and basestations operating according to international standards, such as forexample CDMA (Code Division Multiple Access) and GSM (Global System forMobile Communication).

FIG. 4 illustrates a wireless communication system in which the signalclipping circuit for improved sigma delta analog-to-digital convertersof the present invention may be implemented. Wireless communicationsystem 40 comprises a wireless user terminal (a cellular handset beingillustrated) 42 that communicates with a base station (a cellular basestation being illustrated) 44 over an uplink channel 46 and downlinkchannel 48. The base station and the wireless user terminal unit operatein a similar manner.

Cellular communication in system 40 can be facilitated in Time DomainDuplex (TDD) or in Frequency Domain Duplex (FDD). In Time Domain Duplex(TDD) the communication between wireless user terminal 42 and basestation 44 is on a single channel. Much like a walky-talky, the channelis shared in time by the mobile station transmitter and the base stationtransmitter. A time slot is dedicated to the uplink and another timeslotis dedicated to a downlink. The relative length of the uplink anddownlink time slots can be adjusted to accommodate asymmetric datatraffic. If it is found that downlink data traffic is on average twicethat of uplink, then the downlink time slot is twice as long as theuplink time slot. In Frequency Domain Duplex (FDD) the wireless userterminal 42 and the base station 44 communicate over a pair of radiofrequencies. The lower frequency is the uplink during which the mobilestation sends information to the base station. Both uplink and downlinkare each composed of a signal source, a transmitter, the propagationpath, a receiver and a method of presenting the information. Bothwireless user terminal and base station embody the invention withtransmitters, which convert digital data to analog signals at high speedand with high resolution. The base station could convert the entiremulti-carrier downlink signal to analog for use in a single RFtransmitter. The wireless user terminal is explained in the following.

FIG. 5 presents a top-level block diagram 50 of the wireless userterminal 42. In wireless user terminal 42, radio frequency (RF) signalsare received and transmitted by the RF section 52. In the embodimentillustrated, RF section 52 comprises a duplexer 76 coupling an antenna78 to a receiver 68 and a power amplifier 74. A modulator 72 is coupledto power amplifier 74 and to a synthesizer 70. Synthesizer 70 is furthercoupled to receiver 68. RF section 52 is further coupled to an analogbaseband 54. In the embodiment illustrated, analog baseband 54 comprisesan RF interface 56 and an audio interface 58. A speaker 88 and amicrophone 90 are coupled to audio interface 58. RF interface 56 iscoupled to both receiver 68 and modulator 72 of RF section 52. Theanalog RF interface 56 includes I and Q analog-to-digital converters(ADCs), according to the present invention, and digital-to-analogconverters (DACs), for conversion between the analog and digitaldomains. Audio interface 58 may also include I and Q analog-to-digitalconverters (ADCs), according to an embodiment of the present invention,and digital-to-analog converters (DACs), for conversion between thedigital and analog domains. Analog baseband 54 is further coupled to adigital baseband 60.

In the illustrated embodiment, digital baseband 60 comprises threeelements: digital signal processor (DSP) 62, microcontroller unit (MCU)64 and application specific integrated circuit (ASIC) 66. DSP 62 couplesaudio interface 54 to RF interface 56 and to microcontroller unit (MCU)64. Digital signal processor (DSP) 62 and microcontroller unit (MCU) 64are further coupled to ASIC backplane 66. Microcontroller unit (MCU) 64is further coupled to a user interface 80, which comprises at least auser display 82 and a keyboard 84 (an optional SIM card 86 is alsodisclosed).

The digital signal processor (DSP) 62, provides programmable speechcoding and decoding (vocoder), channel coding and decoding,equalization, demodulation and encryption. The microcontroller unit(MCU) handles level 2 & 3 protocol, radio resource management, shortmessage services, man-machine interface and the real-time operatingsystem. The ASIC backplane 66 performs all chip-rate processing. Whiletop level diagram 50 illustrates RF section 52, analog baseband 54 anddigital baseband 60 as being separate packages or chips, the inventioncontemplates substitution of any of the above with an equivalentfunction, such as an RF function, and/or an analog baseband functionand/or a digital baseband function. The functions will remain the sameeven if the actual implementation varies. The invention furthercontemplates that RF section 52, analog baseband 54 and digital baseband60 may be selectively combined and/or integrated into one or twopackages or chips.

An uplink voice processing chain 46 for a wireless user terminal 42 isillustrated in FIG. 6. This channel includes a CODEC 100 coupling amicrophone 90 to a vocoder 98, a baseband modulator 96 coupling vocoder98 to a digital-to-analog converter 92 at high speed and highresolution. An RF transmitter 94 (part of RF section 52) couples anantenna 78 to digital-to-analog converter 92. Within RF transmitter 94,modulator 72 is implemented as two RF mixers, I and Q driven by thesynthesizer, implemented as an RF local oscillator. RF transmitter CODEC100 includes an audio amplifier (not shown), sigma-deltaanalog-to-digital converter (ADC) (not shown) and a digital filter (notshown) coupled together on one chip. The CODEC receives an analog voicesignal through the microphone and converts it to a digital signal. WhileCODEC 100 is shown as being separate from digital baseband 60, it mayalso be internal to digital baseband 60. CODEC 100 transcodes audiosignals into digital words using the algorithms contained in theVOCODER. This signal is then complex modulated, converted to analog(I&Q) and applied to the transmitter. The transmitter is complexmodulated at the radio frequency assigned to the handset. It uses apower amplifier coupled to the antenna 78 to transmit the digitalsignal, effectively communicating the (digital) voice information to thebase station receiver.

A downlink voice channel 48 for wireless user terminal 42 is illustratedin FIG. 7. This channel includes an RF receiver 102 (part of RF section52) coupling antenna 78 to an analog-to-digital converter (ADC) 104,according to the invention, a vocoder 98 coupling a demodulator 96 to aCODEC 100, and a speaker 88 coupled to CODEC 100. While CODEC 100 isshown as being separate from digital baseband 60, it may also beinternal to digital baseband 60. CODEC 100 transcodes the digital wordsinto analog signals using the algorithms contained in the VOCODER. CODEC100 includes a digital filter, DAC and audio amplifier coupled togetheron one chip. The RF receiver uses an AGC circuit which varies the IFamplifier gain as a function of the received signal. The goal is topresent the analog-to-digital converters (ADCs) with a full-scale analogsignal without distortion and with minimal noise.

The band structure of the cellular system in which the communicationsystem of the present invention operates is composed of tightly packedRF carriers with very high spectral density. As illustrated in FIG. 8,the world's most widely deployed TDMA system is GSM, where theGMSK-modulated carriers are placed on a 200-Khz raster 106 with adjacentchannel signal interference suppressed to −30 dBc at the first adjacentchannel and −60 dBc at the second. The 2-G CDMA system used in America(IS-95) uses QPSK-modulated (at 1.2288 Msps) carriers spaced at 1.25 Mhz108 with very little guard band. Each carrier can be modulated with upto 32 Walsh codes, which are used to separate the users.

FIG. 9 illustrates the spectral definition of the 2G and 3G cellularregulations. The base station transmitter operates on the upperfrequency band. For example, in Europe the base station receives from1900 to 1980 Mhz and transmits from 2110 to 2170 Mhz.

The signal analog-to-digital converter of the present invention can beuse in other applications, such as data communication systems, hard diskdrives, cd players, video displays, and any other application wherethere is a large amount of data that must be converted quickly.

Those skilled in the art to which the invention relates will appreciatethat various substitutions, modifications and additions can be made tothe described embodiments, without departing from the spirit and scopeof the invention as defined by the claims.

1. A wireless user terminal having radio frequency (RF) communicationcapability, comprising: a digital baseband; an RF section; an analogbaseband coupling said digital baseband to said RF section, wherein saidanalog baseband further comprises: a switch having a first lead coupledto an input and a second lead coupled to an analog-to-digital converter(ADC); a signal clipping circuit having a first input coupled to saidfirst input of said switch, a second input coupled to receive a firstreference voltage, a third input coupled to receive a second referencevoltage, and an output coupled to said second lead of said switch. 2.The wireless user terminal of claim 1 wherein said analog-to-digitalconverter is a sigma-delta analog-to-digital converter.
 3. The wirelessuser terminal of claim 1 wherein said wireless user terminal is acellular handset.
 4. The wireless user terminal of claim 1 wherein saiddigital baseband further comprises: a digital signal processor (DSP); amicrocontroller unit (MCU) coupled to said DSP; and an ASIC backplanecoupled to said DSP and said MCU.
 5. The wireless user terminal of claim1 wherein analog baseband comprises an audio interface coupled to saidDSP and to a speaker and a microphone.
 6. The wireless user terminal ofclaim 1 wherein said analog baseband comprises an RF interface coupledto said DSP and to said RF section.
 7. The wireless user terminal ofclaim 1 wherein said analog baseband comprises an audio interfacecoupled to said DSP and to a speaker and a microphone and an RFinterface coupled to said DSP and to said RF section.
 8. The wirelessuser terminal of claim 1 wherein said RF section comprises a duplexercoupling a receiver and power amplifier to an antenna.
 9. The wirelessuser terminal of claim 8 wherein said RF section further comprises amodulator coupling said RF interface to a power amplifier.
 10. Thewireless user terminal of claim 8 wherein said RF section furthercomprises a synthesizer coupled to said modulator and to said receiver.11. The wireless user terminal of claim 1 further including a userdisplay and a keyboard coupled to said digital baseband.
 12. Thewireless user terminal of claim 4 further including a user display and akeyboard coupled to said MCU.
 13. The wireless user terminal of claim 6wherein said analog-to-digital converter is located within said RFinterface.
 14. The wireless user terminal of claim 6 wherein saidanalog-to-digital converter is located within said Audio interface. 15.The wireless user terminal of claim 1 wherein the clipping circuitcomprises a first branch for clipping an output signal at an upperboundary, and a second branch for clipping the output signal at a lowerboundary.
 16. The wireless user terminal claim 15 wherein the firstbranch comprises: a comparator having an output, a first input coupledto receive a first threshold voltage and a second input coupled toreceive the input signal; and a switch coupled to the output of thecomparator, the output voltage of the comparator couples to the switchto open and close the switch.
 17. The wireless user terminal of claim 16wherein the first input of the comparator is a negative input and thesecond input of the comparator is a positive input.
 18. The wirelessuser terminal of claim 15 wherein the second branch comprises: acomparator having an output, a first input coupled to receive a firstthreshold voltage and a second input coupled to receive the inputsignal; and a switch coupled to the output of the comparator, the outputvoltage of the comparator couples to the switch to open and close theswitch.
 19. The wireless user terminal of claim 18 wherein the firstinput of the comparator is a negative input and the second input of thecomparator is a positive input.
 20. A radio frequency (RF) enabledcommunications system, comprising: a base station; and a wireless userterminal capable of communicating with said base station via radiofrequency (RF) communication, said wireless user terminal furthercomprising: a digital baseband; an RF section; an analog basebandcoupling said digital baseband to said RF section, wherein said analogbaseband further comprises: a switch having a first lead coupled to aninput and a second lead coupled to an analog-to-digital converter (ADC);a signal clipping circuit having a first input coupled to said firstinput of said switch, a second input coupled to receive a firstreference voltage, a third input coupled to receive a second referencevoltage, and an output coupled to said second lead of said switch. 21.The radio frequency (RF) enabled communications system of claim 20wherein said analog-to-digital converter is a sigma-deltaanalog-to-digital converter.
 22. The radio frequency (RF) enabledcommunications system of claim 20 wherein said wireless user terminal isa cellular handset.
 23. The radio frequency (RF) enabled communicationssystem of claim 20 wherein said digital baseband further comprises: adigital signal processor (DSP); a microcontroller unit (MCU) coupled tosaid DSP; and an ASIC backplane coupled to said DSP and said MCU. 24.The radio frequency (RF) enabled communications system of claim 20wherein analog baseband comprises an audio interface coupled to said DSPand to a speaker and a microphone.
 25. The radio frequency (RF) enabledcommunications system of claim 20 wherein said analog baseband comprisesan RF interface coupled to said DSP and to said RF section.
 26. Theradio frequency (RF) enabled communications system of claim 20 whereinsaid analog baseband comprises an audio interface coupled to said DSPand to a speaker and a microphone and an RF interface coupled to saidDSP and to said RF section.
 27. The radio frequency (RF) enabledcommunications system of claim 20 wherein said RF section comprises aduplexer coupling a receiver and a power amplifier to an antenna. 28.The radio frequency (RF) enabled communications system of claim 27wherein said RF section further comprises a modulator coupling asynthesizer to said power amplifier and said receiver coupled to saidsynthesizer.
 29. The radio frequency (RF) enabled communications systemof claim 28 wherein an input of said modulator is coupled to an outputof said RF interface and an output of said receiver is coupled to aninput of said RF interface.
 30. The radio frequency (RF) enabledcommunications system of claim 20 further including a user display and akeyboard coupled to said digital baseband.
 31. The radio frequency (RF)enabled communications system of claim 23 further including a userdisplay and a keyboard coupled to said MCU.
 32. The radio frequency (RF)enabled communications system of claim 25 wherein said digital-to-analogconverter is located within said RF interface.
 33. A wireless userterminal having radio frequency (RF) communication capability,comprising: a digital baseband; an RF section; an analog basebandcoupling said digital baseband to said RF section, wherein said analogbaseband further circuitry for providing an analog-to-digital (ADC)function, wherein said circuitry comprises: a first input; a switchhaving a first lead coupled to an first input and a second lead coupledto an analog-to-digital converter (ADC); a second input coupled to saidanalog-to-digital converter (ADC), said second input being furthercoupled to receive an input signal; a signal clipping circuit having afirst input coupled to said first input of said switch, a second inputcoupled to receive a first reference voltage, a third input coupled toreceive a second reference voltage, and an output coupled to said secondlead of said switch.
 34. A wireless user terminal having radio frequency(RF) communication capability, comprising: circuitry for providing adigital baseband function; circuitry for providing an RF function;circuitry for providing an analog baseband function, said circuitry forproviding an analog baseband function being coupled to said circuitryfor providing a digital baseband function and said circuitry forproviding an RF function, wherein said circuitry for providing an analogbaseband function comprises: a first input; a switch having a first leadcoupled to an first input and a second lead coupled to ananalog-to-digital converter (ADC); a second input coupled to saidanalog-to-digital converter (ADC), said second input being furthercoupled to receive an input signal; a signal clipping circuit having afirst input coupled to said first input of said switch, a second inputcoupled to receive a first reference voltage, a third input coupled toreceive a second reference voltage, and an output coupled to said secondlead of said switch.